It is known that a powder, thin film, etc. of carbon cluster molecules each composed of a plurality of carbon atoms linked to form a spherical shape or an ellipsoidal shape having a so-called fullerrenes structure, such as C.sub.60 or C.sub.70, is doped with an impurity atom, such as an alkali metal, to provide an organic conducting material or a superconducting material.
For example, a potassium-doped C.sub.60 film and a rubidium-doped C.sub.60 film have been reported to have conductivity of 500 S/cm and 100 S/cm, respectively (as described in Nature, Vol. 350, p. 320 (Mar. 28, 1991)).
It was also reported that potassium-doped C.sub.60, K.sub.x C.sub.60, shows superconductivity at a critical temperature (hereinafter abbreviated as "Tc") of 18K from the results of microwave absorption and susceptibility measurements and at a Tc of 16K from the results of resistivity measurement (as described in Nature, Vol. 350, p. 600 (Apr. 18, 1991)) and that rubidium-doped C.sub.60, Rb.sub.x C.sub.60, shows superconductivity at a Tc of 28K (as described in Physical Review Letters, Vol. 66, p. 2830 (1991). Cesium-doped C.sub.60, Cs.sub.x C.sub.60, was reported to show superconductivity at a Tc of 30K, and cesium- and rubidium-doped C.sub.60, Cs.sub.2 RbC.sub.60, was reported to show superconductivity at a Tc of 33K (as described in Nature, Vol. 352, pp. 222-223 (Jul. 18, 1991)).
Superconductivity of carbon clusters doped with Ca, an alkaline earth metal, or Sn, a group IVb metal, was also reported. That is, Nature, Vol. 355, p. 529 (Feb., 6, 1992) reported superconductivity of Ca.sub.5 C.sub.60 at a Tc of 8.4K, and Solid State Commn., Vol. 82, No. 3, p. 167 (1992) reported superconductivity of a tin-doped C.sub.60 /C.sub.70 mixture, Sn.sub.x C.sub.60 /C.sub.70, at a Tc of 37K.
Further, the present inventors have ascertained that a carbon cluster doped with indium (In), a group IIIb element, can easily be obtained by simultaneous vacuum evaporation of indium and a carbon cluster by means of a vacuum deposition apparatus, an electron beam epitaxy (MBE) deposition apparatus, etc. and that the amount of the dopant In can be controlled simply by varying the rate of In deposition in the simultaneous vacuum evaporation so that fullerite conducting materials having an arbitrary conductivity, inclusive of from a nearly insulator to a semiconductor-like material, can be prepared with extreme ease and good reproducibility.
However, as has been well known, since an alkali metal element is highly reactive and instable, when a carbon cluster doped with an alkali metal element, e.g., K, Rb or Cs, is exposed to the atmosphere, the dopant element undergoes reaction with oxygen or water in the atmosphere and thereby escapes from the carbon cluster, resulting in disappearance of the above-mentioned characteristics in a short period of time.
This tendency is observed with a carbon cluster doped with other metal elements. For example, the present inventors have revealed that a powder of C.sub.60 doped with Ca, an alkaline earth metal, loses its superconductivity when left in air for 1 hour. According to the literature cited above, the Sn-doped C.sub.60 powder retains its superconductivity when allowed to stand in air for one day, but stability over a longer time remains unclear. The present inventors have additionally revealed that an In-doped C.sub.60 film also loses its characteristics in air within a short time.
A currently widespread means for prevention of deterioration of metal element-doped carbon clusters is to seal them in an inert gas atmosphere. For example, the doped cluster is put in a glass tube containing an inert gas, and the open end of the tube is heat sealed or sealed with an epoxy resin.
However, where the open end of a glass tube is heat sealed, the carbon cluster is likely to undergo deterioration by the heat of the heat sealing. Where sealing is conducted with an epoxy resin, the resin possibly undergoes cracking by changes in environmental temperature due to the difference in thermal expansion coefficient between glass and the resin. If once a crack occurs, oxygen or water in air is no more prevented from entering the glass tube, and deterioration of characteristics cannot be avoided.
Besides, a carbon cluster once sealed into a glass tube as described above is difficult to be integrated into a device because of the difficulty in connecting to outer wiring.